At present, halogen-containing flame retardants (especially brominated flame retardants) are widely used in high-molecular flame retardant materials and have better flame retardant effect. However, it is concluded after in-depth study on fire scenes that, although halogen-containing flame retardants have better flame retardant effect and less additional amount, high-molecular materials using halogen-containing flame retardants will produce a large amount of toxic and corrosive gases and smokes and likely make people suffocate, resulting in more serious harm than the fire itself (Waste Electrical and Electronic Equipment Directive and the Restriction of Directive on the Use of Certain Hazardous Substances in Electrical and Electronic Equipment were implemented by European Union in Jul. 1, 2006.) The development of halogen-free flame retardant printed wiring boards has become a key point of the industry development, and many manufacturers of copper clad laminates have launched their own halogen-free flame retardant copper clad laminates.
In the current industry, phosphorus-containing resins are commonly used to achieve flame retardant effect. However, the introduction of too much phosphorus will increase the water absorption of the substrates and worsen the chemical resistance. In recent years, the development of the use of benzoxazine as the matrix resin in halogen-free substrates has received more and more attention. Benzoxazine is a benzo hexaheterocyclic system consisting of oxygen atoms and nitrogen atoms, and has the feature of ring-opening polymerization. When being polymerized, there is no release of small molecules. After polymerization, a reticular structure similar to phenolic resin is formed. The products have a small solidification shrinkage, a low porosity, better mechanical, electrical and flame retardant properties.
On the other aspect, electronic products develop in the direction of lightness, thinness, shortness, high density, security, and high function with the rapid development of electronic industry, which requires that electronic elements shall have higher signal transmission rate and efficiency, and puts higher performance demands on the printed circuit boards as carriers. Information processing of electronic products is developed in the direction of high speed and multi-function; the application frequency increases continuously, and frequencies higher than 3 GHz will gradually become the mainstream. Besides higher demands on heat resistance to laminate materials, the demands on dielectric constant and dielectric loss value become lower.
The current conventional FR-4 is difficult to meet the use demand on high frequency and high-speed development of electronic products. Meanwhile, the substrate materials, together with electronic components, become one important route for PCB and terminal manufacturers and designers to improve the product performances, rather than play the mechanical support role of traditional sense. High dielectric constant (Dk) will slow down the signal transmission rate, and high dielectric loss (Df) will convert partial signal into heat, resulting in loss in the substrate materials. Thus, the development of high-frequency transmission having a low dielectric constant and a low dielectric loss, especially halogen-free high-frequency substrates has become a key point in the industry of copper clad laminates.
In order to solve the aforesaid problems, CN101684191B discloses a condensate having a lower dielectric constant and dielectric loss obtained by curing epoxy resin with combined benzoxazine, styrene-maleic anhydride and phosphorus-containing curing agents. However, there will unavoidably be many other problems by reducing the dielectric performances of the materials only with styrene-maleic anhydride, especially the effect on cohesiveness since non-polar styrene structural unit in styrene-maleic anhydride (SMA) reduces the polarity of the modified matrix resin and weakens the interaction between the resin and the copper foil. Meanwhile, a large amount of benzene ring structures in SMA increase the brittleness of resin crosslinking network, and produce adverse effect on the bonding property under dynamic conditions, so as to reduce the bonding strength between the substrates and between the substrates and copper foils.
CN100523081C discloses a condensate having a lower dielectric constant and dielectric loss obtained by curing phosphorus-containing, halogen-free and phosphorus-free epoxy resin with combined benzoxazine, styrene-maleic anhydride and other curing agents. Though phosphorus-containing epoxy resin as the main resin may achieve excellent flame retardancy, too much introduction of phosphorus will have a great effect on the water absorption of the substrates, which will necessarily have adverse effects on many other performances of the substrates.
CN103131131A discloses a condensate having a lower dielectric constant and dielectric loss obtained by curing epoxy resin with combined benzoxazine, styrene-maleic anhydride and amine curing agents. Although common benzoxazine may achieve the object of curing epoxy resin and flame retardant, it is difficult to meet the high-frequency high-speed transmission since it has a higher dielectric constant. Though the introduction of the amine curing agent may improve the cohesiveness, its defects of great hydroscopicity and insufficient heat resistance for curing epoxy resin will necessarily have adverse effects on the use in high multilayer circuit boards.
Thus, it is a problem needed to be solved at present how to produce a prepreg and a laminate having a low dielectric constant, a low dielectric loss, and at the same time ensuring excellent chemical resistance.